In the process of analyzing a sample by mass spectrometry (MS), an MS system first ionizes the sample to create analyte ions. The MS system then transfers the ions into a mass analyzer, and the mass analyzer resolves the ions on the basis of the ions' differing mass-to-charge (m/z) ratios. An ion detector measures the abundance of the ions at each m/z ratio detected. The MS system then processes signals outputted by the ion detector to generate mass (m/z) spectra that provide quantitative and qualitative information regarding the components of the sample (e.g., compounds, isomers, elements, etc.).
The mass analyzer operates in a controlled high-vacuum environment, for example at 10−5 to 10−9 Torr. In some MS systems, the ion source (where ionization of the sample is performed) also operates at a vacuum pressure. In other MS systems, such as when coupled to a liquid chromatography (LC) instrument (an LC-MS system), the ion source operates at or around atmospheric pressure. An MS system utilizing an atmospheric pressure ionization (API) source requires an interface between the API source and the evacuated regions of the MS system in which the mass analyzer and other devices are located. The interface needs to effectively isolate the atmospheric-pressure region where the ions are created (the API source) from the evacuated regions where the ions are processed and measured. At the same time, the interface needs to provide a way to efficiently transport the ions from the API source into the evacuated regions after the ions are created.
An ion transfer device in the form of a capillary (i.e., small-bore) tube is often utilized to transfer the ions from the API source into the first vacuum region of the MS system. The capillary tube has a small inside bore, the diameter of which may range from a fraction of a millimeter (mm) to a few millimeters. The capillary tube extends through the boundary between the API source and the first vacuum region, whereby the capillary tube's entrance is exposed to the ionization region of the API source and the capillary tube's exit is exposed to the first vacuum region. Ions and gas in the API source are drawn into the capillary tube's entrance, transported through the capillary tube's bore, and emitted from the capillary tube's exit into the first vacuum region. Ion optics guide the ions further into the MS system and ultimately to the mass analyzer.
The capillary tube may be metal, which allows for fast polarity switching in applications where the MS system is switched between detecting positive ions and negative ions. However, the metal inside wall defining the inside bore, or lumen, of the capillary tube is easily oxidized or subjected to other types of chemical reaction (e.g., deposition and/or erosion) that degrade the performance of the capillary tube and shorten its usable service life.
Alternatively, the capillary tube may be composed of a pure dielectric material (i.e., a bulk dielectric material) such as glass. However, electrostatic surface charges in the bore tend to build up on the dielectric inside wall, and the dielectric material is unable to carry away the charges. This results in reduced ion transmission efficiency and thus loss of ion signal, especially in the case of lower ion masses for which the ion signal may eventually completely vanish. Moreover, this type of dielectric capillary tube is incapable of fast polarity switching.
A dielectric capillary tube may be provided with metal coatings as electrodes at its inlet end and outlet end to allow the application of voltage potential gradient. However, such end-coated capillary tube suffers the same problems in the middle part between the coated ends. Additionally, the metal coatings are subjected to the harsh chemical environment and thus are prone to degradation as noted above.
As another alternative, the capillary tube may be constructed of a dielectric material with an electrically resistive coating on the inside surface of the capillary bore. That is, the coating is a material that is electrically conductive with a high electrical resistance. This approach reduces the problem of the dielectric wall becoming charged, as the resistive material is able to dissipate the charges. The approach may improve ion transport as much as 100-fold in comparison to use of a purely dielectric capillary, and also may enable faster polarity switching. However, the resistive coatings are usually metallic and thus easily degraded by oxidization or other chemical reaction, thereby degrading the performance of the capillary tube. For example, such reactions may change the conductivity of the resistive coating and degrade ion transmission efficiency.
Because a capillary tube becomes degraded due to exposure to the harsh chemical environment, users of MS systems are forced to clean or replace the capillary tube frequently to maintain a consistent ion signal response and stability in the MS system. Each cleaning or replacement of the capillary tube requires shutting down the MS system, cooling down the capillary tube, and in many cases bringing the MS system down to ambient pressure. The cleaning or replacement of the capillary tube may require the MS system to be out of operation for several days, severely limiting productivity.
In view of the foregoing, a need remains for improved ion transfer devices such as capillary tubes.